Adaptations of higher plant cell walls to water loss: drought vs desiccation

Water-deficit stress poses unique challenges to plant cells dependent on a hydrostatic skeleton and a polysaccharide-rich cell wall for growth and development. How the plant cell wall is adapted to loss of water is of interest in developing a general understanding of water stress tolerance in plants and of relevance in strategies related to crop improvement. Drought tolerance involves adaptations to growth under reduced water potential and the concomitant restructuring of the cell wall that allow growth processes to occur at lower water contents. Desiccation tolerance, by contrast, is the evolution of cell walls that are capable of losing the majority of cellular water without suffering permanent and irreversible damage to cell wall structure and polymer organization. This minireview highlights common features and differences between these two water-deficit responses observed in plants, emphasizing the role of the cell wall, while suggesting future research avenues that could benefit fundamental understanding in this area.     

Growing Out of Stress: The Role of Cell- and Organ-Scale Growth Control in Plant Water-Stress Responses

Water is the most limiting resource on land for plant growth, and its uptake by plants is affected by many abiotic stresses, such as salinity, cold, heat, and drought. While much research has focused on exploring the molecular mechanisms underlying the cellular signaling events governing water-stress responses, it is also important to consider the role organismal structure plays as a context for such responses. The regulation of growth in plants occurs at two spatial scales: the cell and the organ. In this review, we focus on how the regulation of growth at these different spatial scales enables plants to acclimate to water-deficit stress. The cell wall is discussed with respect to how the physical properties of this structure affect water loss and how regulatory mechanisms that affect wall extensibility maintain growth under water deficit. At a higher spatial scale, the architecture of the root system represents a highly dynamic physical network that facilitates access of the plant to a heterogeneous distribution of water in soil. We discuss the role differential growth plays in shaping the structure of this system and the physiological implications of such changes.     

Effects of water turbulence on variations in cell ultrastructure and metabolism of amino acids in the submersed macrophyte,Elodea nuttallii (Planch.) H. St. John

The interactions between macrophytes and water movement are not yet fully understood, and the causes responsible for the metabolic and ultrastructural variations in plant cells as a consequence of turbulence are largely unknown. In the present study, growth, metabolism and ultrastructural changes were evaluated in the aquatic macrophyte Elodea nuttallii, after exposure to turbulence for 30 days. The turbulence was generated with a vertically oscillating horizontal grid. The turbulence reduced plant growth, plasmolysed leaf cells and strengthened cell walls, and plants exposed to turbulence accumulated starch granules in stem chloroplasts. The size of the starch granules increased with the magnitude of the turbulence. Using capillary electrophoresis–mass spectrometry (CE‐MS), analysis of the metabolome found metabolite accumulation in response to the turbulence. Asparagine was the dominant amino acid that was concentrated in stressed plants, and organic acids such as citrate, ascorbate, oxalate and γ‐amino butyric acid (GABA) also accumulated in response to turbulence. These results indicate that turbulence caused severe stress that affected plant growth, cell ultrastructure and some metabolic functions of E. nuttallii. Our findings offer insights to explain the effects of water movement on the functions of aquatic plants.     

Expression of the plant cyclin-dependent kinase inhibitor ICK1 affects cell division, plant growth and morphology

The plant CDK inhibitor ICK1 was identified previously from Arabidopis thaliana with its inhibitory activity characterized in vitro. ICK1 displayed several structural and functional features that are distinct from known animal CDK inhibitors. Despite the initial characterization, there is no information on the functions of any plant CDK inhibitor in plants. To gain insight into ICK1 functions in vivo and the role of cell division during plant growth and development, transgenic plants were generated expressing ICK1 driven by the cauliflower mosaic virus 35S promoter. In comparison to control plants, growth was significantly inhibited in transgenic 35S-ICK1 plants, with some plants weighing <10% of wild-type plants at the 3 week stage. Most organs of 35S-ICK1 plants were smaller. There were also modifications in plant morphology such as shape and serration of leaves and petals. The changes were so drastic that 35S-ICK1 plants with strong phenotype no longer resembled wild-type plants morphologically. Analyses showed that increased ICK1 expression resulted in reduced CDK activity and reduced the number of cells in these plants. Cells in 35S-ICK1 plants were larger than corresponding cells in control plants. These results demonstrate that ICK1 acts as a CDK inhibitor in the plant, and the inhibition of cell division by ICK1 expression has profound effects on plant growth and development. They also suggest that alterations of plant organ shape can be achieved by restriction of cell division.     

Advances in functional regulation mechanisms of plant aquaporins: Their diversity,gene expression,localization, structure and roles in plant soil-water relations (Review)

Aquaporins are important molecules that control the moisture level of cells and water flow in plants. Plant aquaporins are present in various tissues, and play roles in water transport, cell differentiation and cell enlargement involved in plant growth and water relations. The insights into aquaporins’ diversity, structure, expression, post-translational modification, permeability properties, subcellular location, etc., from considerable studies, can lead to an understanding of basic features of the water transport mechanism and increased illumination into plant water relations. Recent important advances in determining the structure and activity of different aquaporins give further details on the mechanism of functional regulation. Therefore, the current paper mainly focuses on aquaporin structure-function relationships, in order to understand the function and regulation of aquaporins at the cellular level and in the whole plant subjected to various environmental conditions. As a result, the straightforward view is that most aquaporins in plants are to regulate water flow mainly at cellular scale, which is the most widespread general interpretation of the physiological and functional assays in plants.     

Advances in functional regulation mechanisms of plant aquaporins: their diversity, gene expression, localization, structure and roles in plant soil-water relations (Review)

Aquaporins are important molecules that control the moisture level of cells and water flow in plants. Plant aquaporins are present in various tissues, and play roles in water transport, cell differentiation and cell enlargement involved in plant growth and water relations. The insights into aquaporins' diversity, structure, expression, post-translational modification, permeability properties, subcellular location, etc., from considerable studies, can lead to an understanding of basic features of the water transport mechanism and increased illumination into plant water relations. Recent important advances in determining the structure and activity of different aquaporins give further details on the mechanism of functional regulation. Therefore, the current paper mainly focuses on aquaporin structure-function relationships, in order to understand the function and regulation of aquaporins at the cellular level and in the whole plant subjected to various environmental conditions. As a result, the straightforward view is that most aquaporins in plants are to regulate water flow mainly at cellular scale, which is the most widespread general interpretation of the physiological and functional assays in plants.     

The Importance of Cell Size in the Water Relations of Plants

Several structural changes in cotton (Gossypium hirsutum L.) leaves attendant on development under conditions of water deficit were examined. Cell size was less and cell wall thickness greater in the leaves of stressed plants than in leaves of well-watered plants. A short review of the literature suggested that the lesser cell size is a fairly general observation and that it may contribute to plant resistance to moisture stress. A simple model is developed to investigate the influence of the reduction of cell size on cellular water relations. The predictions which can be drawn from simulations with this model are that smaller cells should maintain turgor to lower values of water potential than larger cells. Rather large changes in cell water relations are predicted for small changes in cell size. These effects are related principally to the changing proportion of cell water which resides in the cell wall and is external to the plasmalemma and the osmotic adjustment system. This prediction is in agreement with several observerations on the behavior of stress-hardened plants and supports the hypothesis that plants or tissues with the smaller cell size will be more tolerant of low water potential.     

The apparent viscosity of the protoplasm of subepidermal stem basis cells of Pisum sativum. Relation to aging, drought tolerance and water stress

Plants of Pisum sativum L. cv. Alaska wilt resistant were subjected to two different water stress regimes under controlled environment conditions: watering was stopped either on the 7th day (early stress) or on the 21st day (late stress) after planting. Plants under the early stress regime developed drought tolerance (adapted), while those under late stress did not. The apparent viscosity of the protoplasm of subepidermal stem basis cells was analyzed by the centrifugation and plasmolysis form method during the entire growth period.
The apparent viscosity of the subepidermal stem basis cells changed with plant age and was highest in 3-week-old plants. In controls the relation of apparent viscosity to age was the same when measured under full turgor and in relaxed state. Under early stress condition, however, the pattern of the viscosity changes with plant age was significantly different for turgescent and relaxed cells. In four week old plants, a higher apparent viscosity was measured in relaxed adapted cells than in relaxed control cells. It is suggested that the higher apparent viscosity is the result of a delayed cell aging.
Apparent viscosity was inversely proportional to soil moisture content and the osmotic potential of the cell sap for the cells of late stress plants, whereas no clear relation was found for the cells of early stress plants. This difference may indicate two mechanisms of viscosity changes: 1) osmotic dehydration of the protoplasm under water stress (passive viscosity change), 2) changes in the amount, hydration or architecture of macromolecules present in the cytoplasm (active viscosity change). Whereas differences in the apparent viscosity between control and stressed cells may not be the cause of drought tolerance, they seem to indicate the development of drought tolerance. Water stress history and plant age were the most critical factors controlling the apparent viscosity changes observed.     

Use of infrared microspectroscopy to determine leaf biochemical composition of cassava in response to Bacillus subtilis CaSUT007

The objective of this study was to investigate the growth stimulating properties of Bacillus subtilis CaSUT007 applied to cassava plants using fourier transform infrared (FTIR) microspectroscopy to monitor the production of cellular components involved in plant growth and development. Cassava stakes treated with CaSUT007 or sterile distilled water were germinated in soil. After incubation for 2 months, CaSUT007 treated plants had higher growth rate and greater biomass than the control. FTIR analysis revealed that the leaves of cassava plants treated with CaSUT007 display FTIR spectra changes in the epidermis and mesophyll tissue. These changes associated with proteins, lipids, and pectins, which are related to changes in plant cell growth and development. FTIR microspectroscopy can be used as a new tool to examine the biochemical changes within the plant tissue. This technique allows us to reveal structural chemical makeup and features of different tissue types.     

Aging in Perennials

Compared with our knowledge of senescence processes in annuals and biennials, relatively little is known about age-related changes in perennials. The study of aging in plants is very complex and there is no consensus in general concepts related to this topic. Furthermore, there is also a problem of scaling up, which makes us wonder whether cells, tissues/organs or whole organisms really age in plants. This is particularly interesting in the case of perennials, which have the ability to make new leaves every year and live for several years or even centuries or millennia. Recent studies indicate that physiological burdens, such as demands on water and nutrient supply, are responsible for reduced growth as plants age. Aside from the extrinsic factors, it is also possible that intrinsic changes in the shoot meristems could occur through repeated cell divisions and could be fixed during plant development, thereby affecting the physiology of leaves that originated from these cells. Additionally, the increased size associated with the aging of woody perennials (trees and shrubs) has also been proposed as a determining factor responsible for the age-related reductions in growth and photosynthetic rates in leaves. This review is aimed at compiling our current understanding of aging in perennials. After defining some fundamental questions and concepts, and introducing the model plants presently used in the study of aging in perennials, the major role meristems play in perenniality and how aging is manifested in the physiology of perennials (changes in phytohormones, water relations, photosynthesis and oxidative stress) are described. Finally, the causes underlying age-related changes in perennials are discussed in detail and a model based on plant plasticity to explain the aging phenomenon in perennials is presented.     

Cell Wall Modifying Proteins Mediate Plant Acclimatization to Biotic and Abiotic Stresses

Cell wall modification is an important aspect of plant acclimation to environmental stresses. Structural changes of the existing cell wall mediated by various cell wall modifying proteins help a plant adjust to environmental changes by regulating growth and policing the entry of biotic agents. For example, accelerated shoot growth during submergence and shading allows some plants to escape these unfavorable conditions. This is mediated by the regulation of wall modifying proteins that alter cell wall structure and allow it to yield to turgor, thus fueling cellular expansion. Regulation of cell wall protein activity results in growth modulation during drought, where maintenance of root growth through changes in wall extensibility is an important adaptation to water deficit. Freeze-tolerant plants adjust their cell wall properties to prevent freezing-induced dehydration and also use the cell wall as a barrier against ice crystal propagation. Cell wall architecture is an important determinant of plant resistance to biotic stresses. A rigid cell wall can fend off pathogen attack by forming an impenetrable, physical barrier. When breached, products released during wall modification can trigger plant defense signaling. This review documents and discusses studies demonstrating the importance of timely cell wall modification during plant stress responses by focusing on a well-researched subset of wall modifying proteins.     

Revealing on hydrogen sulfide and nitric oxide signals co-ordination for plant growth under stress conditions

In the recent times, plants are facing certain types of environmental stresses, which give rise to formation of reactive oxygen species (ROS) such as hydroxyl radicals, hydrogen peroxides, superoxide anions and so on. These are required by the plants at low concentrations for signal transduction and at high concentrations, they repress plant root growth. Apart from the ROS activities, hydrogen sulfide (H₂S) and nitric oxide (NO) have major contributions in regulating growth and developmental processes in plants, as they also play key roles as signaling molecules and act as chief plant immune defense mechanisms against various biotic as well as abiotic stresses. H₂S and NO are the two pivotal gaseous messengers involved in growth, germination and improved tolerance in plants under stressed and non-stress conditions. H₂S and NO mediate cell signaling in plants as a response to several abiotic stresses like temperature, heavy metal exposure, water and salinity. They alter gene expression levels to induce the synthesis of antioxidant enzymes, osmolytes and also trigger their interactions with each other. However, research has been limited to only cross adaptations and signal transductions. Understanding the change and mechanism of H₂S and NO mediated cell signaling will broaden our knowledge on the various biochemical changes that occur in plant cells related to different stresses. A clear understanding of these molecules in various environmental stresses would help to confer biotechnological applications to protect plants against abiotic stresses and to improve crop productivity.     

Evaluation of diel patterns of relative changes in cell turgor of tomato plants using leaf patch clamp pressure probes

Relative changes in cell turgor of leaves of well‐watered tomato plants were evaluated using the leaf patch clamp pressure probe (LPCP) under dynamic greenhouse climate conditions. LPCP changes, a measure for relative changes in cell turgor, were monitored at three different heights of transpiring and non‐transpiring leaves of tomato plants on sunny and cloudy days simultaneously with whole plant water uptake. Clear diel patterns were observed for relative changes of cell turgor of both transpiring and non‐transpiring leaves, which were stronger on sunny days than on cloudy days. A clear effect of canopy height was also observed. Non‐transpiring leaves showed relative changes in cell turgor that closely followed plant water uptake throughout the day. However, in the afternoon the relative changes of cell turgor of the transpiring leaves displayed a delayed response in comparison to plant water uptake. Subsequent recovery of cell turgor loss of transpiring leaves during the following night appeared insufficient, as the pre‐dawn turgescent state similar to the previous night was not attained.     

Effects of Sodium Chloride on Water Status and Growth of Sugar Beet

The effects of sodium chloride on the water status, growth,and physiology of sugar beet subjected to a range of soil waterpotentials were studied under controlled conditions. Sodiumchloride increased plant dry weight and the area, thickness,and succulence of the leaves. It increased the water capacityof the plant, mainly the shoot, but there was no evidence thatit altered the relationships between leaf relative water contentand the leaf water, osmotic, and turgor potentials or changedthe way stomatal conductance and photosynthesis responded todecreasing leaf water potential. The greater leaf expansionin sodium-treated plants is thought to be the consequence ofadjustments made by leaf cells to accommodate changes in ionsand water in a way that minimizes change in water and turgorpotentials. It is also suggested that the greater water capacityof treated plants buffers them against deleterious changes inleaf relative water content and water potential under conditionsof moderate stress.     

土层浅薄地区植物水分来源研究方法

植物水分来源取决于环境中有效水的分布及植物获取水分的能力.旱季,土层浅薄地区土壤水无法满足植物生长的需要,植物能否利用风化基岩层水分是其能否维持正常水分消耗的关键.本文综述了4种土层浅薄地区植物水分来源的研究方法,包括调查和分析植物根系生长与分布特征、监测地表以下各层次水分变化、监测并分析植物体水分指标季节变化以及运用稳定同位素技术区分植物水分来源,并进一步分析了各种方法的优势和局限性及其在我国西南喀斯特地区植物水分来源研究中的应用前景.     

Non-hydraulic regulation of fruit growth in tomato plants (Lycopersicon esculentum cv. Solairo) growing in drying soil

Tomato (Lycopersicon esculentum cv. Solairo) fruit growth, fruit mesocarp and leaf epidermal cell turgor, and fruit and leaf sub-epidermal apoplastic pH were monitored as plants were allowed to dry the soil in which they were rooted. Soil drying regimes involved splitting the root system of plants between two halves of a single pot separated by a solid impervious membrane to form a split-root system. Plants were then allowed to dry the soil in both halves of the pot (a soil-drying (SD) treatment) or water was supplied to one-half of the pot (a partial root-drying (PRD) treatment), allowing only one-half of the root system to dry the soil. A well-watered control treatment watered the soil on both halves of the pot. The rate of fruit growth was highly correlated with the soil water content of both sides of the SD treatment and the dry side of the PRD treatment. Soil drying caused a significant restriction in fruit growth rate, which was independent of any changes in the turgor of expanding fruit mesocarp cells in the PRD treatment. By supplying water to half of the root system, the turgors of mesocarp cells were maintained at values above those recorded in well-watered controls. The turgor of leaf epidermal cells exhibited a similar response. The pH of the sub-epidermal apoplastic compartment in leaves and fruit increased with soil drying. The dynamics of this increase in leaves and fruit were identical, suggesting free transport of this signal from shoot to fruit. Fruit growth rate and sub-epidermal pH within the fruit showed a strong correlation. The similarity of fruit growth response in the SD and PRD treatment, suggests that tomato plants respond to a discrete measure of soil water status and do not integrate measures to determine total soil water availability. The results of this study are not consistent with Lockhartian models of growth regulation in expanding fruit of a higher plant. A non-hydraulic, chemical-based signalling control of fruit growth in plants growing in drying soil is proposed.     

Channelling auxin action: modulation of ion transport by indole-3-acetic acid

The growth hormone auxin is a key regulator of plant cell division and elongation. Since plants lack muscles, processes involved in growth and movements rely on turgor formation, and thus on the transport of solutes and water. Modern electrophysiological techniques and molecular genetics have shed new light on the regulation of plant ion transporters in response to auxin. Guard cells, hypocotyls and coleoptiles have advanced to major model systems in studying auxin action. This review will therefore focus on the molecular mechanism by which auxin modulates ion transport and cell expansion in these model cell types.     

Cell elongation as an inseparable component of growth in terrestrial plants

The review is dedicated to the role of cell elongation in plant growth and morphogenesis. The ratios of cell division to elongation, cell competence for the initiation of elongation, main features of the metabolism of elongating cells, and physiological processes realizing elongation have been considered on the examples of seed germination and growth of roots, stems, and leaves. A special attention was paid to the vacuole as a specific feature of plant cells, pathways of its formation, and its role in maintenance of ion and water homeostasis in the elongating cell. The plant can modify its morphology according to changes in the environmental conditions via cell elongation.     

植物生理学研究中的压力探针技术

压力探针技术是一种用来测定微系统中压力大小和变化的新技术。其最初被设计用于直接测定巨型藻类的细胞膨压。随着操作装置的进一步微型化和精密化, 后来被应用于测定普通高等植物细胞膨压及其它水分关系参数。该技术的发展建立在一系列相应的流体物理学理论基础上。通过这些物理学公式的计算, 该技术能测定跨细胞膜或器官的水分运输速度以及它们的水力学导度; 测定溶液中水分和溶质的相对运输速度以及它们之间的相互影响; 还可以测定细胞壁的刚性等。目前压力探针技术已成为植物生理学和生态学领域研究中的多用途技术。它可以在细胞水平上原位测定水分及溶质跨膜运输及分布情况, 这对于阐明水通道功能具有极其重要的意义。此外, 木质部压力探针技术是目前唯一可以直接测定导管或管胞中负压的工具。该技术还可以用于单细胞汁液的样品采集, 结合微电极技术测定导管或其它细胞中的pH值、离子浓度以及细胞膜电位。本文重点介绍该技术使用的基本原理和相应的理论基础, 并详细地描述了操作过程中的技术和技巧。     

Cell elongation as an inseparable component of growth in terrestrial plants

The review is dedicated to the role of cell elongation in plant growth and morphogenesis. The ratios of cell division to elongation, cell competence for the initiation of elongation, main features of the metabolism of elongating cells, and physiological processes realizing elongation have been considered on the examples of seed germination and growth of roots, stems, and leaves. A special attention was paid to the vacuole as a specific feature of plant cells, pathways of its formation, and its role in maintenance of ion and water homeostasis in the elongating cell. The plant can modify its morphology according to changes in the environmental conditions via cell elongation.